1. Field of the Invention
The present invention relates to a cleaning device and an image forming apparatus using the same and more particularly to a structure for cleaning an image carrier included in an image forming apparatus.
2. Description of the Background Art
An electrophotographic process is one of conventional processes available for forming an image corresponding to a document or image information and disclosed in, e.g., U.S. Pat. No. 2,297,091 and Japanese Patent Publication Nos. 49-23910 and 43-24748. It is a common practice with the electrophotographic process to use an image carrier having a photoconductive layer. The photoconductive layer is exposed imagewise or scanned by a light beam to form a latent image representative of a document or image information. In the case of development using dry toner, the latent image is developed by the dry toner to become a toner image. The toner image is transferred to a sheet or recording medium and then fixed thereon by, e.g., heat and pressure.
Developing devices are generally classified into a wet-process developing device and a dry-process developing device. The wet-process developing device uses a developing liquid consisting of an insulative, organic liquid and various kinds of pigments and dyes finely dispersed in the liquid. The dry-process developing device uses a developer implemented as dry toner consisting of natural or synthetic resin and carbon black or similar colorant dispersed in the resin. The dry-process developing device develops a latent image by using any one of a cascade method, a magnet brush method, a powder cloud method and other conventional methods. The developer for the dry-process developing device is either one of toner only and a toner and carrier mixture.
Today, to meet the increasing demand for higher image quality, the grain size of toner for development is decreasing. Particularly, when a latent image is formed in the form of dots by digital processing, toner with a small grain size is often used to enhance reproducibility and sharpness. Toner with a small grain size is taught in, e.g., Japanese Patent Laid-Open Publication Nos. 1-112253, 2-284158 and 7-295283. These documents describe the mean grain sizes of toner and the amounts of toner grains having a mean grain size of 5 μm or below as well as the distributions of such toner grains specifically. The mean grain size of 5 μm or below is essential for high definition, high resolution images. Such toner grains faithfully deposit on a latent image without blurring or otherwise disfiguring it.
An edge effect, which is one of problems particular to image formation, is conspicuous when toner with a mean grain size of 5 μm or below, but can be obviated if the toner contains a particular number percent of grains having grain sizes of 5 μm and above. Therefore, when use is made of toner with a mean grain size of 5 μm or below and containing 60 number percent to 80 number percent of grains with a grain size of 5 μm or above, high definition and high resolution are achievable. Such toner, however, is apt to bring about defective cleaning, as will be described hereinafter.
In an electrophotographic image forming apparatus, after a toner image formed by toner on an image carrier has been transferred to a sheet, the surface of the image carrier is cleaned to remove toner left on the image carrier. One of conventional cleaning devices uses a cleaning blade pressed against the surface of the image carrier for scraping off the residual toner. This type of cleaning device, however, cannot adequately remove the residual toner and makes images defective when use is made of toner with a small grain size for the following reasons.
Toner for development contains not only resin, which is the major component, but also various additives for different purposes. Likewise, sheets contain various additives. Although such additives each are effective for a particular purpose, they are separated from the toner and sheets due to repeated image formation and deposit on the image carrier. It is difficult for the cleaning blade to fully remove the additives, which are fine grains, so that toner with a small grain size accumulates on the surface of the image carrier little by little. This part of the toner is blocked by the fine grains of additives and cannot be easily scraped off despite the action of the cleaning blade.
The toner accumulated on the toner grains left on the drum eventually form masses that do not transmit light in an expected manner. Consequently, stains intermittently appear on the background or the white portion of an image and therefore appear in the resulting toner image in the form of a black stripe, making the toner image defective.
Another problem with toner with a small grain size is that the amount of charge to deposit on the individual toner grain increases, increasing adhesion acting between the toner and the image carrier. This makes it difficult for the cleaning blade to block the toner grains and thereby causes the toner grains moved away from the cleaning blade to form black stripes in the background of an image.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-138798 and 9-50215, Japanese Utility Model Publication No. 7-33260, and Japanese Patent No. 3,155,421.